1. Field of the Invention
The present invention relates to the field of liquid crystal displaying techniques, and in particular to a liquid crystal medium composition for use in liquid crystal display.
2. The Related Arts
Liquid crystal used in twisted nematic (TN) or super twisted nematic (STN) liquid crystal display is positive liquid crystal. The liquid crystal molecules have a long axis that is arranged parallel to a substrate surface when no electricity is applied. The orientation of the liquid crystal molecules on the substrate surface is determined by rubbing direction of an alignment layer (which is usually made of polyimide). The alignment directions of two substrate surfaces are perpendicular to each other, whereby the molecules of the liquid crystal layer exhibit a continuously twisted arrangement from one substrate surface to another substrate surface. When a voltage is applied, the long axis of the liquid crystal molecule tends to align in the direction of electric field. Drawbacks of the TN or STN liquid crystal display are small viewable angle and severe brightness difference and color difference at large view angles. A compensation film must be applied to correct these problems and this increases the manufacture cost of display devices.
Multi-domain vertical alignment (MVA) thin-film transistor liquid crystal display (TFT-LCD) provides an excellent solution to the view angle limitation that the TN or STN displays are subjected to and negative liquid crystal and vertical alignment film are used. When no voltage is applied, the long axis of liquid crystal molecule is perpendicular to the substrate surface. Application of voltage would cause the liquid crystal molecule to tilt, making the long axis of the liquid crystal molecule aligning in a direction perpendicular to the electric field. To overcome the view angle problem, a pixel is divided into multiple domains and liquid crystal molecules are caused to tilt in different directions so that the display, when viewed from different directions, can provide similar viewing effect. Several ways can be adopted to have liquid crystal molecules of different domains of a pixel oriented in different directions. As shown in FIG. 1A, the first way is to form polymer bumps 108 on upper and lower glass substrates 102, 103 by means of exposure development in order to cause a pre-tilt angle for liquid crystal molecules around the polymer bumps 108 and thus guiding the liquid crystal molecules 106 to tilt to predetermined directions. As shown in FIG. 1B, the second way is to form an ITO pixel electrode 304 on upper and lower glass substrates 302, 303 in such a way as forming a predetermined pattern so that an electric field so induced shows a predetermined tilt angle thereby controlling the orientations of the liquid crystal molecules 306 in different domains. This technique is often referred to as patterned vertical alignment (PVA). As shown in FIG. 2, the third way is to form ITO slits 501 on a TFT side of a lower glass substrate 500, while ITO of an upper glass substrate 502 remains full. Polymerizable monomers 508 are added in the liquid crystal medium. An electric field is first applied to cause tilting of the liquid crystal molecules 506 and then ultraviolet ray is applied to irradiate the panel so as to polymerize the monomers to form polymer bumps 510 that guide tilting of the liquid crystal molecules and are deposited on the surfaces of the upper and lower glass substrates 500, 502 to achieve an effect of alignment. This technique is referred to as polymer stabilized vertical alignment (PSVA).
The formation of polymer bumps in the PSVA technique is a phase separation process. Before polymerization, the monomers are small molecules and show good compatibility with the liquid crystal medium composition. After being irradiated by ultraviolet light and undergoing polymerization, the monomers form high molecules that are separated from the liquid crystal medium composition so as to form polymer bumps that are insoluble in the liquid crystal medium. These are polymer bumps that show an effect of alignment. However, the known techniques do not show how to control the degree of polymerization of reactive mesogenic (RM) monomers and the size of the polymerized particles. Such characteristics are of vital influence for the effect of alignment and optic performance of panels. Taking the conventionally used material as an example, if the intensity of the irradiation light is weak, then the reaction rate of RM is slow and if the intensity of irradiation light is strong, the reaction rate of RM is fast, which may lead to the formation of excessively large particles of polymerization, whereby the alignment effect is poor, undersize spots may be formed, leakage in dark state may occur, and contrast is reduced. Thus, the key issue of the PSVA technique is controlling the reaction of the monomers in order to form bumps that are of suitable sizes and uniformly distributed and thus preventing poor alignment of liquid crystal and occurrence of bright spots in dark state of liquid crystal panels and obtaining excellent optic performance of the panel, such as high contrast and fast response.
Referring to FIGS. 3-5, the known techniques use only a single type of monomer, making it easy to cause undesired situations, such as easy occurrence of bumps formed by excessively large particles due to variation of light irradiation conditions, so that undersize bright spots may be seen in a dark state of a PSVA panel and the contrast of the panel is reduced. Thus, further improvement may be made on the liquid crystal medium composition for the PSVA techniques.